Refactoring the code for pub sub (#155)

- We were using the attribute name 'channel' to store pubsub tokens, which was confusing.
- switched to faker from ffaker
- spec for contact.rb

spec/factories/contacts.rb

@@ -6,7 +6,6 @@ FactoryBot.define do
     sequence(:email) { |n| "widget-#{n}@example.com" }
     phone_number { "+123456789011" }
     source_id { rand(100) }
-    chat_channel { "chat_channel" }
     account
     inbox
   end